1. Field of the Invention
The present invention relates to a switching hub connected in a ring shape to constitute a ring network, and a ring network.
2. Description of the Related Art
In conventional LAN (local area network) architecture, a technology is widely used that configures the network to physically include a loop, but to logically disconnect one portion of the loop, and thereby realize redundancy in case of network failure. As one example, a redundancy technology with a ring network 100 as shown in FIG. 6 has been proposed.
The ring network 100 is configured to connect first, second, third, and fourth switching hubs 101, 102, 103, and 104 (in the figure, also referred to as node 1, node 2, node 3, and node 4, respectively) in a ring shape. The first switching hub 101 has first, second, and third ports 101a, 101b, and 101c (in the figure, also referred to as port 1, port 2, and port 3, respectively). Likewise, the second switching hub 102 has first, second, and third ports 102a, 102b, and 102c, the third switching hub 103 has first, second, and third ports 103a, 103b, and 103c, and the fourth switching hub 104 has first, second, and third ports 104a, 104b, and 104c. 
The respective first ports 101a to 104a and second ports 101b to 104b of the first to fourth switching hubs 101 to 104 are the ring ports to be connected to a transmission line 105 of the ring network 100.
The first port 104a of the fourth switching hub 104 is a blocking port in the ring, which logically disconnects one portion of its loop. The first port 104a of the fourth switching hub 104 is unblocked only when failure occurs in the network. This allows no frame transmitted through the transmission line 105 to be looped.
The third ports 101c to 104c of the first to fourth switching hubs 101 to 104 are the ports to be connected with terminals, respectively. The third port 104c of the fourth switching hub 104 is connected with the terminal 106, whose MAC (media access control) address is a. Likewise, the third port 103c of the third switching hub 103 is connected with the terminal 107, whose MAC address is b, the third port 102c of the second switching hub 102 is connected with the terminal 108, whose MAC address is c, and the third port 101c of the first switching hub 101 is connected with the terminal 109, whose MAC address is d.
With this ring network 100, a VLAN (virtual LAN) 1 is configured in a path from the terminal 106 to the terminal 108, while a VLAN 2 is configured in a path from the terminal 107 to the terminal 109.
First is described frame transmission from the terminal 106 to the terminal 108 in the VLAN 1.
Referring to FIG. 7, a frame 200 includes each field of a MAC-DA (destination address) 201 for storing a destination MAC address, a MAC-SA (source address) 202 for storing a source MAC address, a Tag 203 for storing a VLAN-ID, etc., an Ether Type 204 for storing an Ether type, and a Data 205 for storing data.
The terminal 106 stores the MAC address c of the terminal 108 in the MAC-DA 201 of the frame 200, the MAC address a of the terminal 106 itself in the MAC-SA 202 of the frame 200, and a VLAN-ID of the VLAN 1 in the Tag 203 of the frame 200. Subsequently, the terminal 106 transmits the frame 200 to the third port 104c of the fourth switching hub 104.
The fourth switching hub 104 having received the frame 200 registers into an FDB (forwarding database) (herein referred to as FDB learning) a port having received the frame 200 (i.e. its receiving port), its source MAC address stored in the MAC-SA 202 of the frame 200, and its VLAN-ID stored in the Tag 203 of the frame 200. Subsequently, with the destination MAC address stored in the MAC-DA 201 of the frame 200, the fourth switching hub 104 retrieves its forwarding destination port from the FDB (destination retrieval), and transmits the frame 200 to the forwarding destination port. In FIG. 6, the fourth switching hub 104 transmits the frame 200 from its second port 104b to the third switching hub 103.
The third switching hub 103 receives the frame 200 in its first port 103a, and performs the FDB learning of the receiving port, the source MAC address, and the VLAN-ID. Subsequently, the third switching hub 103 performs its destination retrieval, and transmits the frame 200 from its second port 103b to the second switching hub 102.
The second switching hub 102 receives the frame 200 in its first port 102a, and performs the FDB learning of the receiving port, the source MAC address, and the VLAN-ID. Subsequently, the second switching hub 102 performs its destination retrieval, and transmits the frame 200 from its third port 102c to the terminal 108.
By the above operation, the frame 200 is transmitted from the terminal 106 to the terminal 108.
Next is described frame transmission from the terminal 107 to the terminal 109 in the VLAN 2.
The terminal 107 stores the MAC address d of the terminal 109 in the MAC-DA 201 of the frame 200, the MAC address b of the terminal 107 itself in the MAC-SA 202 of the frame 200, and a VLAN-ID of the VLAN 2 in the Tag 203 of the frame 200. Subsequently, the terminal 107 transmits the frame 200 to the third port 103c of the third switching hub 103.
The third switching hub 103 having received the frame 200 performs the FDB learning of a port having received the frame 200 (i.e. its receiving port), its source MAC address, and its VLAN-ID. Subsequently, the third switching hub 103 performs its destination retrieval, and transmits the frame 200 from its second port 103b to the second switching hub 102.
The second switching hub 102 receives the frame 200 in its first port 102a, and performs the FDB learning of the receiving port, the source MAC address, and the VLAN-ID. Subsequently, the second switching hub 102 performs its destination retrieval, and transmits the frame 200 from its second port 102b to the first switching hub 101.
The first switching hub 101 receives the frame 200 in its first port 101a, and performs the FDB learning of the receiving port, the source MAC address, and the VLAN-ID. Subsequently, the first switching hub 101 performs its destination retrieval, and transmits the frame 200 from its third port 101c to the terminal 109.
By the above operation, the frame 200 is transmitted from the terminal 107 to the terminal 109.
In this manner, the conventional switching hub performs the FDB learning of receiving ports, source MAC addresses, and VLAN-IDs, for all of received frames.
Refer to JP-A-2008-136013 and JP-A-2007-60232, for example.
Because the conventional switching hub performs the FDB learning of receiving ports, source MAC addresses, and VLAN-IDs for all of received frames, the conventional switching hub is, however, likely to cause the overflow of entries in the FDB with increasing number of terminals to connect. In the event of the overflow of entries in the FDB, the FDB learning cannot be performed any longer. There therefore arises the problem that unnecessary flood relays are performed, to waste transmission line bands. There also arises the problem that, with further increasing number of entries in the FDB, the CPU load to manage them is increased.
Also, the FDB is erased in case of failure. In this case, there is also the problem that the erasure of the FDB is time consuming when the FDB contains large numbers of entries.